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8. Modbus TCP server 8.1. Modbus TCP frames
Modbus TCP frames consist of a header and a Modbus request.
Header format:
Byte Description Comments
0 high order
1
Transaction
identifier low order
2 high order
3
Protocol
identifier low order This identifier always equals 0.
4 high order
5 Length of data low order
Number of bytes in the Modbus request +1. The frame length is
always less than 256 bytes, the value of the significant byte
therefore equals 0.
6 Destination identifier (Unit ID)
7 Modbus request function
code
The frame header returned by the VF-MB1/S15 server is identical to that of the frame
sent by the client.
8.2. Drive Modbus servers
The destination identifier (Unit ID) is used to access drive Modbus TCP servers:
Unit ID Modbus TCP server Accessible parameters
0-248 Variable speed drive See the VF-MB1/VF-S15 instruction manual.
251 Option board
255 IO Scanner See the “IO Scanner” section.
8.3. List of Modbus functions supported
Function code Modbus name Description Size of data
03 Read Holding Registers Read N output words 63 words max.
06 Write Single Register Write one output word -
16 (0x10) Write Multiple Registers Write N output words 63 words max.
23 (0x17) Read/Write Multiple Registers Read/write N words
(IO Scanning)
20/20 words max.
43 (0x2B) Read Device Identification Identification -